The shear plate interferometer provides a fast and simple manual means of measuring defocus, astigmatism and coma in a laser beam. A normal, commercially available shear plate interferometer typically comprises a glass shear plate which has a slight vertical wedge in it and is mounted in an L-shaped bracket so as to be maintained in a vertical position. When a horizontal laser beam is made obliquely incident on this shear plate, a finite derivative of its wavefront is taken in the direction of shear and the reflected beams from its front and back surfaces interfere with each other in a well known manner to produce horizontal fringes. The distance between adjacent fringes depends on the wedge angle. A horizontally sheared interferogram thus obtained measures the laser beam aberrations which vary in the horizontal plane but does not record aberration components which vary only in the vertical direction. Thus, it may be possible to balance astigmatism against defocus in the horizontal plane but there may be left a vertically varying cylindrical wavefront error.